"I was not able to have any chance when I was returning through the whole match," Nadal said. "When that's happening against a player like Novak, you are dead."
Yet Nadal seemed to put the setback behind him in light of an extraordinary comeback season that sees him returning to the top ranking for the first time since July 2011.
"I am back at the No. 1, something that I really thought that I would never have the chance to be back there," he said. "I'm playing one of the best seasons of my career, probably one of the more emotional years, if not the most for so many facts."
Williams' serve proved decisive in the women's final, in which both players were feeling the effects of a long season. Jankovic needed treatment after the third game of the second set for what appeared to be a sore back, returning only to lose the next five straight games.
"I served pretty well tonight. It was important to serve well because she's a good returner and it's always a good plus to do that," said Williams, who won 74 percent of the points when she made her first serve. "The first game was a little hard but then I got more into a rhythm tonight."
Williams won the first women's event in Beijing in 2004 and said her second win was an unexpected bonus at the end of a year in which she has now won 10 titles, including two Grand Slams.
"I really didn't expect to come and win this tournament. I just wanted to do well, she said.
This year's tournament was plagued by bad pollution in the Chinese capital, but Williams said it hadn't affected her game. Djokovic also brushed aside its impact on the event.
"It's not ideal in terms of pollution," Djokovic said. "Yes, we've been talking about the weather conditions, but it is what it is, it's something that has been the same for the last few years that I've been coming back here."